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                            <h1>Disk Scheduling Algorithms</h1>
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                            <li><a href="#line1">Introduction</a></li>
                            <li><a href="#line2">FCFS</a>
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                                  <li><a href="#line2_1">Description</a></li>
                                  <li><a href="#line2_2">Advantages</a></li>
                                  <li><a href="#line2_3">Disadvantages</a></li>
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                            <li><a href="#line3">SSTF</a>
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                                  <li><a href="#line3_1">Description</a></li>
                                  <li><a href="#line3_2">Advantages</a></li>
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                            <li><a href="#line5">LOOK</a>
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                                  <li><a href="#line5_1">Description</a></li>
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                          <li><a href="#line6">C-SCAN/C-LOOK</a>
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                                            <p>In multi-programmed systems, one of the most important tasks is scheduling the use of shareable resources by various processes. Disk Scheduling is done by the operating systems to schedule IO requests arriving for disk.
                                            </p>

                                            <p>The file system may reside entirely on a single disk, or it may spread out on a number of disks. Thus all processes that ask for disk IO services are competing for access to the same physical disk or set of physical disks. (Note that all processes must do at least one burst of disk IO to initially load a program to run.) Any given disk can perform only one access at a time. Thus if there are several processes requesting for an access, the operating system must establish some order for the service of requests.</p>

                                            <h4>Types</h4>
                                            <p>There are 6 types</p>
                                            <ol>
                                                <li>First In First Out (FIFO)</li>
                                                <li>Shortest Seek Time First  (SSTF)</li>
                                                <li>SCAN</li>
                                                <li>LOOK</li>
                                                <li>Circular SCAN (C-SCAN)</li>
                                                <li>Circular LOOK (C-LOOK)</li>
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                                <h3 class="dark-text">First In First Out<a href="#top">#back to top</a><hr></h3>
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                                <h4 id="line2_1">Description</h4>
                                <p>This is the simplest approach among the available, but way too simple to be used. As the name suggests, the requests are served in the same order as they arrive.</p>
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                                <h4 id="line2_2">Advantages</h4>
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                                    <li>Simple to use and understand.</li>
                                    <li>No indefinite postponing of service to a request. (No starvation)</li>
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                                <h4 id="line2_3">Disadvantages</h4>
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                                    <li>Does not try to improve the seek time</li>
                                    <li>May not provide the best possible service</li>
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                                <h3 class="dark-text">Shortest Seek Time First<a href="#top">#back to top</a><hr></h3>
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                                <h4 id="line3_1">Description</h4>
                                <p>The header moves to the request which is nearest to the current position of the header.  So the seek of every request is calculated in advance in the queue and then scheduled according to the calculated seek time.
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                                <h4 id="line3_2">Advantages</h4>
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                                  <li>Average response time is less</li>
                                  <li>Throughput is more.</li>
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                                  <li>There is a cost to calculate the seek time in advance</li>
                                  <li>Can lead to starvation for a request if it has higher seek time as compared to incoming requests.</li>
                                  <li>The response times vary a lot as SSTF favors requests with less seek time.</li>
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                                <h3 class="dark-text">SCAN<a href="#top">#back to top</a><hr></h3>
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                                <p>This algorithm is also known as the elevator algorithm, because it works the same way an elevator services requests in a building. As a result the requests at the mid-range are serviced more and those arriving behind the disk ark will have to wait.
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                                <h4 id="line4_2">Advantages</h4>
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                                  <li>High Throughput</li>
                                  <li>The response time varies less</li>
                                  <li>Efficient and neat</li>
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                                <h4 id="line4_3">Disadvantages</h4>
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                                  <li>Long waiting time for requests for disk locations just visited by the disk arm.
                                  </li>
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                                <h3 class="dark-text">LOOK<a href="#top">#back to top</a><hr></h3>
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                                <h4 id="line5_1">Description</h4>
                                <p>This algorithm is very similar to SCAN algorithm, except the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there itself. Thus it prevents the extra delay which occurs due to unnecessary traversal to the end of the disk.</p>
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                                  <li>The extra seek time to traverse to the end is prevented</li>
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                            <h4>Output</h4>
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                            <div class="col-md-12 left-align">
                                <h3 class="dark-text">C-SCAN/C-LOOK<a href="#top">#back to top</a><hr></h3>
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                                <h4 id="line6_1">Description</h4>
                                <p>C stands for circular. The algorithms are very similar to SCAN/LOOK except that the disk serves requests in one direction and jumps to the beginning of the disk when it reaches the last track. This produces a more uniform response time and maybe more efficient overall since a single large jump maybe faster than several smaller ones. (Note that
When it reaches the furthest end, there will be proportionally fewer requests near where it is because it has just been servicing that end of the disk.) </p>
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